The present invention relates generally to implantable medical devices and to a method for depositing and affixing solids onto implantable medical devices. More particularly, the present invention relates to a radiopaque implantable device, such as a stent, and to a method for depositing and affixing radiopacifiers onto intravascular or intraductal implant devices.
In a typical percutaneous transluminal coronary angioplasty (PTCA) for compressing lesion plaque against the artery wall to dilate the artery lumen, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced through the vasculature until the distal end is in the ostium. A dilatation catheter having a balloon on the distal end is introduced through the catheter. The catheter is first advanced into the patient""s coronary vasculature until the dilatation balloon is properly positioned across the lesion.
Once in position across the lesion, a flexible, expandable, preformed balloon is inflated to a predetermined size at relatively high pressures to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile, so that the dilatation catheter can be withdrawn from the patient""s vasculature and blood flow resumed through the dilated artery. While this procedure is typical, it is not the only method used in angioplasty.
In angioplasty procedures of the kind referenced above, restenosis of the artery often develops which may require another angioplasty procedure, a surgical bypass operation, or some method of repairing or strengthening the area. To reduce the likelihood of the development of restenosis and strengthen the area, a physician can implant an intravascular prosthesis, typically called a stent, for maintaining vascular patency. A stent is a device used to hold tissue in place or to provide a support for a vessel to hold it open so that blood flows freely. Statistical data suggests that with certain stent designs, the restenosis rate is significantly less than the overall restenosis rate for non-stented arteries receiving a PTCA procedure.
A variety of devices are known in the art for use as stents, including expandable tubular members, in a variety of patterns, that are able to be crimped onto a balloon catheter, and expanded after being positioned intraluminally on the balloon catheter, and that retain their expanded form. Typically, the stent is loaded and crimped onto the balloon portion of the catheter, and advanced to a location inside the artery at the lesion. The stent is then expanded to a larger diameter, by the balloon portion of the catheter, to implant the stent in the artery at the lesion. Typical stents and stent delivery systems are more fully disclosed in U.S. Pat. No. 5,514,154 (Lau et al.), U.S. Pat. No. 5,507,768 (Lau et al.), and U.S. Pat. No. 5,569,295 (Lam et al.).
Stents are commonly designed for long-term implantation within the body lumen. Some stents are designed for non-permanent implantation within the body lumen. By way of example, several stent devices and methods can be found in commonly assigned and common owned U.S. Pat. No. 5,002,560 (Machold et al.), U.S. Pat. No. 5,180,368 (Garrison), and U.S. Pat. No.5,263,963 (Garrison et al.).
Patients treated by PTCA procedures, even when implanted with stents, however, may suffer from restenosis, the coronary vessel collapsing or becoming obstructed by extensive tissue growth, also known as intimal hyperplasia, at or near the site of the original stenosis. Clinical studies have indicated that anti-proliferative drug therapy or intravascular low-dose radiation therapy after balloon angioplasty or an atherectomy procedure can prevent or reduce the rate of restenosis caused by intimal hyperplasia.
One approach for performing low-dose intravascular radiotherapy is using a treatment catheter with a low-intensity radiation source. Another approach uses a low-intensity implantable radioactive device such as a radioactive stent with either beta emitting or low energy gamma-emitting radioisotopes.
Intravascular or intraductal implantation of a radioactive stent generally involves advancing the stent on a balloon catheter or a similar device to the designated vessel/duct site, properly positioning the stent at the vessel/duct site, and deploying the stent by inflating the balloon which then expands the stent radially against the wall of the vessel/duct. Proper positioning of the stent requires precise placement of the stent at the vessel/duct site to be treated. Visualizing the position and expansion of the stent within a vessel/duct area is usually done using a fluoroscopic or x-ray imaging system.
Generally, the implantable stent is made radioactive prior to being inserted into the patient. To make a stent radioactive, a number of techniques are used in the field. For example, a beta-emitting or low energy gamma-emitting radioisotope may be implanted or alloyed into a metal from which the stent is made. The radioisotope may also be coated onto the surface of the stent using an electroplating process. Furthermore, the stent may be made radioactive through neutron activation in a nuclear reactor or similar facility.
Each of these techniques has certain disadvantages including poor and/or non-uniform adhesion of the radioisotope to the surface of the stent, fabrication difficulties with respect to radiation exposure of workers during the manufacturing process, and the risk of generating considerable quantities of undesired isotopes from the neutron activation process which may continue to affect the irradiated tissue long after the desired restenosis treatment period is over. Another significant shortcoming associated with current methods of making a stent radioactive is that these methods are complex and require the performance of many sequential processing steps, which greatly increase the radioactive stent manufacturing cost.
A requirement for any clinically useful stent is that it should have good visibility under fluoroscopic x-ray illumination so that the position of the stent during the implantation procedure is visible to the physician performing the procedure. Since implantable radioactive stents are generally made of metal or metal alloys such as 316L stainless steel or nickel-titanium alloy, such as nitinol, they are not readily visible under fluoroscopic illumination. To make these, and other, non-radioactive, stents manufactured from non-radiopaque materials visible in an x-ray, radiopaque markers are typically attached onto the stent using a number of techniques. One current technique involves applying a coating of a radiopaque marker material such as gold or tantalum onto the stent, or selected portions of the stent, using an electroplating process. Another technique involves soldering or brazing a radiopaque marker material at specific locations onto the stent. Generally, radiopaque markers are soldered at the longitudinal ends, that is, the most proximal and most distal portions of the stent.
A number of shortcomings or disadvantages are associated with the prior art devices and techniques for attaching radiopaque markers onto radioactive or non-radioactive stents. Other current radiopaque markers that are attached within the surface of the stent may impair the expansion capability of the stent. Another disadvantage with current radiopaque marker technology is that, when viewed under fluoroscopic illumination, the radiopaque markers may provide poor or no indication of whether the stent is fully extended. Another significant shortcoming associated with current methods of attaching a radiopaque marker material onto a radioactive or non-radioactive stent is that these methods can be tedious, imprecise, and require the performance of many sequential processing steps, which greatly increase the stent manufacturing cost. Moreover, deposition techniques such as electroplating or sputter coating of the radiopaque materials may completely and uniformly coat the stent, thus altering the surface of the stent so that tissue and fluids within vessel lumens are exposed to a material other than stainless steel. Further, the coating may crack or fatigue when flexed.
The invention provides for improved stent designs and methods for depositing a radiopaque material on an implantable medical device, such as a stent. The stents and methods described all provide for the application of radiopacifiers to render the stent or other intraductal medical device radiopaque, either in whole or in part, thus allowing the use of fluoroscopy to assist in placing the stent or medical device at a desired location in the lumen of a vessel or duct.
A stent or intraductal medical device is provided that has a layer of radiopaque material deposited on the surface of the stent. The radiopaque materials to be deposited on the stent or intraductal medical device materials known in the art of radiopaque markers, such as silver, gold, platinum or tantalum, or other materials that are compatible with implantation in a body lumen or duct and which are visible under fluoroscopy or other body vessel/organ imaging system.
In one embodiment, the radiopaque layer may be contiguous over the surface of the stent. In an alternative embodiment, the surface of the stent may be formed having a plurality of cavities, or microdepots, distributed over the outer surface of the stent. Such a stent is rendered opaque by deposition of radiopaque material atoms in the cavities or microdepots. The microdepots may be formed over the entire outer surface of the stent or medical device, or they may be formed in only selected areas of the device, such as in areas adjacent the distal and proximal ends of the device. Even if microdepots are formed over the entire outer surface of the device, the radiopaque materials may be applied to only selected microdepots. For example, radiopaque material may be deposited only in microdepots adjacent the distal and proximal ends of the device. Deposition of radiopaque material in microdepots located on the outer surface of the device is advantageous in that the radiopaque materials are not exposed to the blood or ductal fluid stream flowing through the interior of the stent or medical device. This helps prevent any deleterious effect on the blood or ductal fluid caused by the radiopaque material.
In one embodiment, the radiopaque materials are deposited on the surface of the stent or medical device by dipping or immersing the stent or medical device into a mixture or solution of radiopaque material atoms and a suitable solvent or suspension agent. Such a solution or mixture may include, for example, phosphoric acid, Freon or other solvent. In another approach, the radiopaque material atoms may be suspended in a polymer solution having material characteristics, such as viscosity or wetting properties, that suspend the radiopaque material atoms in the polymer solution while coating the atoms with the polymer.
The radiopaque material atoms may be applied to the surface of the stent or medical device using a variety of methods, such as dipping or immersion. The entire stent or medical device may be dipped or immersed either in whole, or in part. For example, only the areas of the stent or medical device adjacent to the distal and proximal ends of the stent or medical device may be dipped or immersed in the mixture or solution containing the radiopaque material atoms.
Alternatively, where the stent or medical device includes cavities or microdepots formed on the outer surface of the stent or medical device, the mixture or solution containing the radiopaque material atoms may be deposited in the cavities or microdepots using micro-injection. In this method, the mixture or solution containing the radiopaque material atoms is injected into the cavities or microdepots covering the outer surface of the stent or medical device, or the atoms may be injected into cavities or microdepots in selected areas of the stent or medical device.
When the radiopaque material solution or mixture has coated the stent or medical device, excess radiopaque material solution or mixture may be removed from the stent or medical device by centrifuging or shaking the stent or medical device. Centrifuging is particularly advantageous where the radiopaque material atoms have been deposited on a stent or medical device having cavities or microdepots, since the centrifugal force operating on the device while being centrifuged assists distribution of the radiopaque solution across the device. Moreover, the solution stripped from the stent or medical device may be recycled and reused, thus minimizing loss of material and reducing cost.
In another embodiment of the present invention, the coated stent or medical device may be heated to remove excess solvent or solution and/or to bind the radiopaque material atoms on the surface of the stent or medical device. The heating process may be accomplished using various methods of applying heat in a controlled manner to the coated stent or medical device, such as using a thermal oven, an inert gas plasma, exposing the coated device to an electric arc, or by subjecting the radiopaque atoms coating the stent or medical device to low power exposure from an excimer or other suitable laser.
Where high power, high temperature processes are used to remove excess solvent or solution, the polymer in the mixture may be selected so that it completely incinerates into volatile combustion products during the heating process. Polymers such as polyurethanes and polyolefins, and other suitable polymers, that combust completely at temperatures in the range of 600 to 1000 degrees centigrade could thus be removed from the surface of the stent or medical device, leaving little or no residue.
In yet another embodiment of the present invention, the radiopaque material atoms may be deposited on the surface of the stent or medical device using an electrodeposition process. In this embodiment, the stent or medical device is attached to a cathode or negative terminal of an electrical current source and dipped or immersed into a positively charged ion mixture or solution of radiopaque material atoms. When current flow is initiated, the ions are attracted to the cathode and coat the surface of the stent or medical device.